Commingling and mixing of K-rich basaltic (absarokitic) and partially crystallized, strongly peraluminous, S-type monzogranitic-rhyodacitic magmas have been documented in the 23.8 Ma Antauta hypabyssal complex of the Picotani Group, Puno, southeastern Peru. Magma mixing generated intermediate, andesitic rocks hosting small volumes of SiO2-poor (>8.4 wt %), Fe oxide-rich melt, with TiO2 (>30 wt %) and P2O5 (>23 wt %) contents corresponding to those of the genetically problematic nelsonite ore type. Microscopic spheres of this composition are embedded in rhyolitic glass surrounding both extensively melted, sieve-textured plagioclase xenocrysts derived from the rhyodacitic magma and cognate orthopyroxene phenocrysts exhibiting replacive Fe-rich mantles. The andesites also contain up to 15 modal percent of strongly resorbed, granite-derived, quartz xenocrysts with glass-orthopyroxene-clinopyroxene coronas. Intense, quasipervasive silicification of the fractionating mafic magma is inferred to have displaced the hybridized melt composition into the stable liquid immiscibility region of the Na2O+K2O+MgO+Al2O3-FeO+TiO2+MnO+CaO+P2O5 - SiO2 - H2O system. Some Fe-Ti-P–rich bodies have 30 to ca. 55 wt percent SiO2 and exhibit broadly linear trends of major oxides versus SiO2, but the more SiO2-deficient spheres exhibit extreme compositional scatter, implying the existence of complex immiscibility fields within the parental, protonelsonitic melt domain. In contrast, the predominant, apparently conjugate, silicate glass (70–78 wt % SiO2) is relatively consistent in composition and equivalent to a syenogranite and/or rhyolite. The extreme local variability in the composition of the oxide-rich melt bodies may directly reflect their formation in a magma-mixing environment in which temperature varied over at least 200° to 300°C. Such a compositional range, also apparent in large-scale nelsonite occurrences, indicates that strong fractionation of mafic melts under conditions precluding ferroan spinel crystallization may not be a prerequisite for oxide-silicate immiscibility, and hence nelsonite formation. Specular hematite is widely disseminated in the glassy andesites, commonly clustered around resorbed quartz xenocrysts, and microscopic bodies of illite-montmorillonite-chlorite occur in contact with unaltered glass in unveined andesites, implying endogenous sources of, respectively, iron and hydrothermal fluid. Moreover, the Upper Paleozoic clastic sedimentary envelope of the Antauta center at Pucacancha hosts hydrothermal breccias and veins rich in hematite, barite, quartz, chalcedony, and epidote, with lesser anhydrite and traces of U-, Th-, REE-rich allanite, chalcopyrite, and molybdenian scheelite. This unambiguously hydrothermal mineralization is analogous to that of the iron oxide-copper-gold clan, and we propose that the development of some deposits of this type may have been triggered by the formation and subsequent vesiculation of immiscible Fe-rich oxide melt during the mixing of mafic and felsic magmas.

The St. Lawrence deposit is a large wollastonite skarn (ca. 9 Mt @ 41,3 wollastonite) adjoining the gabbroic-to-syenitic Leo Lake pluton in the granulite facies Frontenac Terrane of the Grenvillian Central Metasedimentary Belt. It occurs within a horseshoe-shaped horizon of quartzite, open to the east. The skarn extends over 1.2 km, and comprises wollastonite-dominant, wollastonite-clinopyroxene, and diverse quartz-feldspar-sulphide (-titanite) layers, interbanded with, and strongly folded within, the quartzite. As in the world-class wollastonite deposits of the Willsboro district, New York, no carbonate protolith is preserved, and calcite is a negligible constituent of the mineralized zones. Wollastonite development is nonetheless ascribed to metasomatism of a metacarbonate unit by magmatogene, silica-rich, CO2-poor (XCO2 < 0.05) fluids, at P = 350 MPa and ca. 560&DEG;C, or, alternatively, 550 MPa and ca. 630&DEG;C. Several high-grade zones, averaging >40% but locally exceeding 80%, of translucent, white, tabular wollastonite grains, with lengths of 0.2 cm to 5 cm, have been delimited by drilling. Preliminary beneficiation studies yield high aspect-ratio and high-purity (low Fe and Mn) concentrates. St. Lawrence compares favourably to other known prospects and operating wollastonite deposits: having good chemical and physical properties, low iron content, and high aspect ratios (>20:1). In addition, the deposit has a favourable size and location, and will benefit from a low Canadian dollar. Canada currently has no wollastonite production.

The Hawley (Mountain Grove) wollastonite prospect (ca. 2.8 Mt, 35% wollastonite) is a potentially economic skarn located in the Sharbot Lake amphibolite facies terrane in the Grenvillian Central Metasedimentary Belt, southeastern Ontario. Wollastonite is associated with varying proportions of clinopyroxene, garnet, and calcite. Skarn development is ascribed to the incursion of magmatogene, silica-rich, CO2-poor (XCO2 < 0.3) fluids at temperatures of ca. 550 degreesC to 650 degreesC, and pressures of ca. 400 MPa (Grammatikopoulos, 1999).

Solid - liquid - vapor fluid inclusions in spodumene in rare-element granitic pegmatites commonly contain one or more high-birefringence minerals. Optical examination and laser Raman spectroscopic analyses of over 500 crystal-rich fluid inclusions in spodumene from Bikita and Kamativi, Zimbabwe, and Bernic Lake (Tanco), Manitoba, indicate that the high-birefringence phase is zabuyelite (Li2CO3). Laser Raman spectra were also obtained from fluid inclusions in a wafer of spodumene from the Tanco pegmatite reported to contain the type samples of diomignite (Li2B4O7)- In every fluid inclusion, the high-birefringence phase was shown to be zabuyelite; no phase yielding the Raman spectrum of Li2B4O7 was observed. Petrographic analysis indicates that,he inclusions are secondary in origin and are trapped within healed fractures and cleavage planes. Essentially all fluid inclusions exhibit extensive necking after the crystallization of solid phases. The dominant solid phases are quartz and zabuyelite. Other minerals such as cookeite. calcite, a cesium-rich phase, apatite and several unidentified minerals were found in fewer than 5% of the fluid inclusions examined. An estimate of the bulk composition of the fluid entrapped within spodumene at Tanco was obtained by averaging the contents of several hundred coeval fluid inclusions, and the composition of individual fluid inclusions was determined using laser Raman spectroscopy, synchrotron X-ray fluorescence and laser ablation - inductively coupled plasma - mass spectrometry. The results indicate that the inclusions in spodumene, like those in the quartz component of SQUI, entrapped a low-salinity (ca. 7 wt.% NaCl equiv.), alkali-rich, aqueous carbonic fluid. We propose that the solid - liquid - vapor inclusions in spodumene from Tanco, Bikita and Kamativi are the products of a reaction between the low-density aqueous carbonic fluid and the host spodumene during the late stages of pegmatite evolution; therefore, they do not represent the products of a complex borosilicate melt as suggested previously.

The Kennack Gneiss comprises a suite of interlayered mafic and felsic igneous rocks that intruded the 397 Ma Lizard Ophiolite Complex, Cornwall, at 376.4±1.7 Ma (U–Pb single-zircon date) and were shortly thereafter ( c. 370 Ma) metamorphosed to the amphibolite facies. Weakly deformed examples at the type-locality of Kennack Sands reveal net-veining of the mafic by the felsic component, dispersed enclaves of the former in the latter and flame-like interfingering of the two, features indicative of magmatic commingling and mixing. The silicic components of the Kennack Gneiss range from granodiorite to syenogranite and from metaluminous to peraluminous, the least silicic rocks being the most peraluminous. They were generated through anatexis of a predominantly metasedimentary crustal source having trace element and isotopic compositions comparable to those of the Devonian Gramscatho Group of South Cornwall. The mafic component of the gneiss includes members (group 1) which are weakly enriched in light REE, have minor negative Ta, Nb, P and Ti anomalies in extended trace element plots, and display primitive, time-corrected 143 Nd/ 144 Nd and 87 Sr/ 86 Sr values, and others (group 2) which are more strongly enriched in all incompatible trace elements, exhibit more prominent negative Ta, Nb, P and Ti anomalies, and have isotopic ratios intermediate between group 1 mafic components and the associated felsic gneiss. Group 2 represents mixtures of group 1 and granitic melts, whereas group 1 rocks are comparable to EMORB and were probably generated through partial melting of a weakly enriched mantle source. Emplacement of two commingled magmas into the base of the Lizard ophiolite, one representing the products of crustal anatexis, implies that the ophiolite was removed from its oceanic setting and was in the process of obduction in the Givetian.

In representative, run-of-mine samples from the Panel and New Quirke mines of the Elliot Lake uranium district, Ontario, brannerite, a metamict uranous titanate, occurs in microscopic grain-aggregates which display wide variations, both in textural habit and in the relative proportions of brannerite, titania polymorphs and/or uraniferous titania. As is widely documented, much of the brannerite occurs as trellis-like arrays of laths and needles pseudomorphing rutile or anatase. In some cases, the laths and needles are cemented by coffinite. Brannerite, with an average composition of (U 0.629 Th 0.039 Ca 0.20 ) (Ti 2.199 Fe 0.13 )O 69 contains ≤ 3 wt% Th and is distinctly Ti-rich relative to the ideal composition, UT i O 6 . Much of the Si reporting consistently in electron microprobe analyses (between 1 wt% and 5 wt%) is tentatively attributed to contamination by the quartz-sericite matrix, and a minor proportion is attributed to the presence of coffinite intergrowths. Uraniferous titania (average partial composition by weight: 10.8% U, 0.3% Th, 32.9% Ti) is considered to represent an intermediate stage in the conversion of titania to brannerite. The leaching behaviour of brannerite in its different modes of occurrence wasstudied qualitatively by the rotating-disc (polished section) method in H 2 SO 4 and HCl solutions at 25° to 70°C. Redox potential was not measured.The progress of the dissolution of the individual brannerite crystallites and grain-aggregates was monitored by optical and scanning electron microscopes. Regardless of the process conditions, the rate-controlling step appears to be the initial leach-pit formation. These pits expand radially though the aggregate as leaching proceeds. This dissolution mode is independent of the relative amounts of brannerite and titania, i.e the type of grain-aggregate. Secondary coffinite intergrowths are readily leachable and enhance the overall leaching kinetics of brannerite by accelerating leach-pit formation. These data provide, for the first time, information on the controls on the leaching of the uraniferous phases which dominate the lower-grade ores in the Elliot Lake district.

In representative, run-of-mine samples from the Panel and New Quirke mines of the Elliot lake uranium district, Ontario, brannerite, a metamict uranous titanate, occurs in microscopic grain-aggregates which display wide variations, both in textural habit and in the relative proportions of brannerite, titania polymorphs and/or "uraniferous titania". As is widely documented much of the brannerite occurs as trellis-like arrays of laths and needles pseudomorphing rutile or anatase. In some cases, the laths and needles are cemented by coffinite. Brannerite, with an average composition of (U0.629Th0.039Ca0.20) (Ti2.199Fe0.13)O-69 contains less than or equal to 3 wt% Th and is distinctly Ti-rich relative to the ideal composition, UTi2O6. Much of the Si reporting consistently in electron microprobe analyses (between 1 wt% and 5 Wt%) is tentatively attributed to contamina lion by the quartz-sericite matrix, and a minor proportion is attributed to the presence of coffinite intergrowths. "Uraniferous titania" (average partial composition by weight: 10.8% U, 0.3% Th, 32.9% Ti) is considered to represent an intermediate stage in the conversion of titania to brannerite. The leaching behaviour of brannerite in its different modes of occurrence was studied qualitatively by the rotating-disc (polished section) method in H2SO4 and HCl solutions at 25 degrees to 70 degrees C. Redox potential was nor measured The progress of the dissolution of the individual brannerite crystallites and grain-aggregates was monitored by optical and scanning electron microscopes. Regardless of its morphology and texture (laths or needles, reticulate or blocky), brannerite is not readily leachable. Regardless of the process conditions, the rate-controlling step appears to be the initial leach-pit formation. These pits expand radially through the aggregate as leaching proceeds. This dissolution mode is independent of the relative amounts of brannerite and titania, i.e. the type of grain-aggregate. Secondary coffinite intergrowths are readily leachable and enhance the overall leaching kinetics of brannerite by accelerating leach-pit formation. These data provide, for the first time, information on the controls on the leaching of the uraniferous phases which dominate the lower-grade ores in the Elliot lake district.

Copper ore grades of the Cuajone, Quellaveco, and Toquepala porphyry Cu(-Mo) deposits, situated at 3,000 to 4,000 m a.s.l. on the Pacific slope of the Cordillera Occidental of southernmost Peru (lats 17-degrees-02'-17-degrees-15' S), have been markedly increased by supergene sulfide enrichment. Following the emplacement of the hypogene mineralization in the early Eocene (52-57 Ma) as the terminal stage in the development of the Toquepala Group continental volcano-plutonic terrane, this Andean transect was gradually reduced by erosion in a semiarid climate to a low altitude topography; unroofing of the deposits had taken place by the mid-Oligocene. The initiation of cordilleran uplift at ca. 25 to 26 Ma, accompanied by the episodic eruption of felsic ash-flow tuffs at the oceanward front of the volcanic arc, led to more rapid erosion and the conversion of the mid-Tertiary landscape into the subplanar Altos de Camilaca surface. The regionally extensive pediplain constitutes the major landform component of the present precordillera surrounding the porphyry centers; its final configuration was attained at ca. 18 to 19 Ma. Ash-flow tuff eruption was widespread and frequent at this time.Supergene enrichment began in the late Oligocene during the progressive lowering of topography and continued through the more abrupt water-table depression resulting from the latest Oligocene to early Miocene uplift. However, the distribution of chalcocite, sensu lato, in the three porphyry deposits and the local postmineralization landforms and volcanic histories differed significantly during the latter interval. At Toquepala and, to a lesser extent, Quellaveco, the landform regimes were dominated by open valleys and ignimbrite blanketing was short-lived, whereas the exposed Cuajone deposit was both the site of aggressive early Miocene valley incision and the deposition of an unusually thick, in part welded, ash-flow tuff at 22.8 +/- 0.7 Ma. As a result, the enrichment blanket at Cuajone remained relatively thin and was even partially eroded in the early Miocene, whereas the blankets at Quellaveco and Toquepala were thickened.Continued strong uplift in the mid-Miocene (ca. 8-15 Ma) generated the array of apron and terrace pediments of the Multiple Pediment stage which dominates the lower cordilleran slopes and, in the mineralized areas, led to the deepening of existing valleys and the development of new fluvial channels. Ignimbrite eruption persisted throughout the Miocene. Again, local regimes of erosion and volcanism proved inimical to supergene activity at Cuajone, but enrichment continued at Quellaveco and, particularly, Toquepala, where there is no record of later Miocene ash-flow accumulation. Uplift in this period was apparently also most extensive in the vicinity of the Toquepala deposit, which experienced the formation of a deep chalcocite blanket, while the extant enrichment zone at Quellaveco was thickened.The development of enriched assemblages containing "massive" chalcocite had clearly terminated by 13.1 +/- 0.4 Ma at Cuajone and by 9.5 +/- 0.5 Ma at Quellaveco; the timing of later enrichment at Toquepala is less constrained, but a mid-Miocene age is probable. Supergene upgrading of the three Peruvian deposits was contemporaneous with that of copper mineralization in northernmost and northern Chile, as at Chuquicamata and La Escondida, and in the Copiapo mining district, in all of which areas a late Oligocene to mid-Miocene age has been inferred for supergene alteration. This major and regionally developed metallogenic episode took place during and between two major periods of cordilleran uplift, under semiarid climatic conditions. The termination of intense enrichment along this 2,000-km stretch of the Cordillera Occidental in the late Miocene was a direct result of marked climatic desiccation, which not only reduced the overall supergene activity but focused fluvial erosion, leading to the incision of steep-walled canyons less favorable for supergene alteration than the earlier subplanar landforms.

Scheelite-molybdenite stockwork mineralization constitutes one component of the Lake George polymetallic (Sb-W-Mo-Au-base metal) deposit, a complex hydrothermal center of Late Silurian (ca. 412 m.y.) age in the Fredericton trough of the northern Appalachians. The stockwork, hosted by Silurian graywackes, in part calcareous, is spatially and temporally related to a postkinematic cupola of biotite monzogranite, and its formation overlapped in time with the emplacement of monzogranitic porphyry dikes. Mineralogical and textural evidence indicates that contact metamorphism associated with the cupola had ceased before the initiation of W-Mo mineralization and that it occurred, at pressures of less than 1.75 kb, in two stages: a peak stage (T > 600 degrees C), evident only in rocks of pelitic composition; and a lower temperature reequilibration (T < 500 degrees C), recorded in rocks of both pelitic and marly compositions.The W-Mo deposit comprises three different scheelite- and/or molybdenite-bearing veinlet types. Type 1 bodies, the earliest formed, are calc-silicate (granditic garnet, wollastonite, clinopyroxene, and calcic amphibole) quartz veinlets, with ubiquitous Ca and H metasomatic alteration envelopes. Mineralogical and fluid inclusion relationships indicate that the fluids ranged in temperature from 550 degrees to 228 degrees C and that temperature decreased away from the cupola. The succeeding type 2 veinlets comprise quartz and lesser amounts of perthitic alkali feldspar, muscovite, calcite, scheelite, molybdenite, and pyrite. Fluid inclusion evidence shows that mineralization dominantly occurred from 400 degrees to 175 degrees C, under a confining pressure of 1.3 kb. Higher grade scheelite and molybdenite deposition was focused in a lower temperature zone, to the north of the cupola, in which CO 2 effervescence occurred. Type 3 veinlets, the last to form, consist of prehnite, molybdenite, and quartz and represent a volumetrically minor mineralization type.In both type 1 and 2 systems, scheelite and molybdenite deposition appears to have been controlled by decreasing temperature and increasing pH. Temperature was a function of distance from the cupola for both veinlet types, but the controls on pH were specific to each. Thus, the pH of type 1 fluids was controlled by wall-rock interaction (H metasomatism), whereas that of type 2 fluids was controlled by CO 2 effervescence.The economic stibnite-quartz veins (Scratch et al., 1984) occupy fractures which transect, and therefore, postdate all stages of W-Mo mineralization.

The Jardin arsenian Cu-Ag deposit of the Copiapo mining district, northern Chile, is in many respects representative of the volcanic-associated, disseminated, cupriferous manto mineralization characteristic of this segment of the central Andean orogen. Containing ca. 1 to 2 million metric tons of ore with a grade of 1.75 percent Cu and 140 g/metric tons Ag, the crudely stratiform deposit is hosted by the upper, brecciated and unwelded part of a approximately 75-m-thick, continental rhyolitic ignimbrite and by an overlying approximately 2-m succession of tuffaceous lacustrine sedimentary rocks. The ignimbrite represents the local basal unit of the Paleocene Hornitos Formation. The reduced, ore-hosting, sedimentary horizons were deposited in an ephemeral saline pond and comprise coarse and fine breccias, tuffaceous sandstones and silt-stones, and calcareous-carbonaceous shales containing thin coaly seams and accumulations of plant debris.The greater part of the mineralization occurs as disseminations, discontinuous veinlets, thin concordant lenses and, most strikingly, cylindrical bodies, 1 to 8 mm in diameter, oriented perpendicular to bedding. Chalcocite, bornite, and tennantite (ca. 1-4.3% Ag) are the major ore minerals and are associated with minor sphalerite, digenite, covellite, chalcopyrite, wit-tichenite, native silver, acanthite, stromeyerite, mckinstryite, jalpaite, and the ruby silvers. Pyrite and marcasite are subordinate constituents of the ore and widely display evidence of replacement by the Cu and Ag minerals. Mineralization in both the ash flow and sediments is associated with moderate argillic and carbonate alteration. The hanging wall of the orebody lies within a red fanglomerate, encroachment of which terminated the accumulation of the lacustrine strata.The sulfidic pipes commonly display concentric mineral zonation suggestive of the successive, in part open-space, precipitation of pyrite and the Cu-Ag minerals. Textural evidence suggests that at least the introduction of pyrite into these bodies occurred prior to the deposition of the immediately overlying strata. The pipes, both individually and in aggregate, do not resemble rhizoliths or dewatering structures. More probable origins are as Scolithus sp. lebensspuren or as gregarious hydrothermal conduits directly reflecting the upward penetration of ore-forming fluids. Sulfide-free pipes are extremely rare, and we favor the latter interpretation.The Jardin mineralization displays features characteristic of several ore deposit types, particularly of the red-bed and epithermal clans. Although the temporal relationships of Cu-Ag mineral deposition and lacustrine sedimentation are uncertain and direct evidence of the conditions of mineralization unavailable, our preferred genetic model involves the convective flow of metal-bearing ground waters through the permeable upper zone of the ignimbrite into the mantling lacustrine strata, during both the fumarolic stage of the tuff and the deposition of the sediments. If this be the case, the emplacement of the disseminated mineralization may have taken place in less than a century; the temporal constraints would be reduced if the Cu-Ag minerals formed largely through replacement of early diagenetic pyrite. The hydrothermal fluids are inferred to have been Cl rich, low temperature, neutral to weakly alkaline, and with an f (sub O 2 ) exceeding the hematite-magnetite buffer; they probably derived their metals and arsenic from nearby, weathered, Upper Cretaceous base and precious metal deposits exposed by the sub-Hornitos Formation erosion surface. Ore deposition occurred in response to reduction, neutralization and moderate cooling on contact with the roof zone of the ignimbrite and the carbon- and sulfur-rich sediments; there is no evidence for sulfide replacement of organic detritus.The Jardin manto may perhaps best be interpreted as an unusual, Ag-rich, red-bed copper deposit nucleated in part by an ash flow cooling on the floor of an intermontane basin. However, the Paleocene epoch in the Copiapo district saw the development of numerous Ag-Cu-As epithermal vein systems, and the deposit could also represent an analogous center in which ore deposition was controlled by a sequence of permeable reduced sediments rather than by faults.

The original host rocks and form of the Minto copper deposit (lat 62 degrees 36' N; long 137 degrees 15' W) have been disguised by a succession of postmineralization events. These were: (1) regional metamorphism in the Early to Middle Triassic which generated the foliated and isoclinally folded fabric of the deposit and mobilized the sulfides, destroying all primary features except, probably, the mineral zonation (bn-cp-mt/cp-bn/cp + or - py); (2) the Late Triassic intrusion (ca 203 m.y.) of the Klotassin granodiorite, which engulfed and dispersed the deposit; (3) endoblastic potash metasomatism, as the intrusion cooled, forming orthoclase porphyroblasts throughout the deposit and its wall rock--chemical equilibrium was attained between silicates in the ore zone rocks and in the wall rocks; (4) recrystallization of biotite throughout the ore and wall rocks, an event recorded by two concordant K-Ar ages (177 + or - 9 and 180 + or - 9 m.y.; Early Jurassic); (5) cataclastic deformation, possibly related to the rapid uplift of the Klotassin suite during Laberge Group sedimentation (Early Jurassic), which fractured feldspars, strained quartz, arid probably sheared orthoclase porphyroblasts in the ore zone; and (6) intrusion of pegmatite and alaskite dikes, probably related to Middle Jurassic or younger intrusive groups of the Yukon Crystalline Terrane. Subsequently, following extensive erosion, the deposit was faulted, possibly in the Eocene, intruded by andesitic dikes related to the Eocene or younger Carmacks Group volcanics, and affected by supergene chlorite-sericite-hematite, kaolinite, and laumontite alteration.The genesis of the deposit remains in doubt. Whereas the highest grade (Cu-, Ag-, and Au-rich) quartz and K-feldspar-bearing rocks could have originated as a zone of silicification and potash silicate alteration associated with a pre-Klotassin granodiorite hydrothermal deposit, a sedimentary (i.e., red-bed copper) origin cannot be ruled out.

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Supplementary data to a paper by R. G. Taylor and K. F. G. Hosking (see also this Bibliography Vol. 34, No. 6, 02 E70-17947)